Method and apparatus for controlling electrical discharges



Feb, 19, 1963 c. w. HANKS ETAL 3,078,388

METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES Filed Oct. 27. 1958 s Shee'Es-Sheet 1 PUMPS VACUUM CHAMBER JHOPTM/G 6/?60/7' (555 06-2) ld M0 07 60 3d //VPU7' F 1953 v c. w. HANKS ETAL v 3,073 3 METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES Filed Oct. 27, 1958 s Sheets-Sheet z VoLTAGE Summ Fe 19,196 c. w. HANKS Em 3, 8,

METHOD AND APPARATUS FOR CONTROLLING ELECTRICAL DISCHARGES Filed Oct. 27, 1958 3 Sheets-Sheet 3 MOLTENA/VODE L-r CATHOOE Zen/AL NUDE :16-5

(Ms/r PES/STA/VCQ/ FIG-4 w \7 E b 3 Furs/w: V (lawke's/srmvcs) 2 w 1 b g 0 r DLFTANCE "4 5 l Q JNVENTOR5 United States Patent ()fifice 3,078,388 Patented Fair. 19, 1963 Delaware Filed Oct. 27, 1958, Ser. No. 769,927 17 Claims. (Cl. 315-107) This invention relates to the control of electrical discharges used for heating, melting, and otherwise treating materials in a high vacuum by electron bombardment; and its chief object is to maintain the power expended in such a discharge and the voltage developed across it within desired ranges, irrespective of variations in operating conditions, such as may result from the irregular release of gases or vapors from the treated material, or other changes that may occur.

In the type of treatment to which the present invention is primarily directed, the discharge to be controlled is established within a constantly-pumped vacuum chamber, which is maintained at an average absolute pressure of the order of one micron of mercury or less. The dis charge occurs from a heated, thermionic cathode to an anode, which is usually and preferably the material to be treated but may under certain circumstances be a crucible containing the material. A certain amount of gaseous matter is preferably introduced into the discharge near the anode, sometimes from an external source but usually by the evolution of vapors and gases from the heated material, so that a pressure gradient exists from the discharge outward into the body of the vacuum chamber, and the gas density is greatest immediately adjacent to the anode. The gas in the immediate vicinity of the electrical discharge is believed to be highly ionized; probably, for the most part, by secondary emission of electrons from the anode, although ionization by primary electrons and by photons may occur to some extent. The discharge is diffuse and luminous; gas focusing concentrates it upon the molten anode surface, over which it spreads fairly uniformly.

In large-scale, high-power operation, ionized gas near the anode may become so highly conductive that there is relatively little voltage drop across or through it, and it forms a plasma which may be considered a virtual anode, much closer to the cathode than the physical anode formed by the bombarded material. In the vicinity of the cathode, however, the gas density is much lower, and the resistivity of the gaseous medium is sufiiciently high that several thousand volts can be maintained between the cathode and the virtual anode when the discharge is properly regulated and controlled as herein explained. Such a discharge tends to break down into a low-voltage arc; the present invention inhibits such breakdown, so that the desired, moderately high-voltage discharge can be maintained and controlled at higher power levels than heretofore.

Some positive ions escape from the plasma and partly neutralize, or may even overneutralize, the negative spacecharge of the electronic current. Owing to their low mobility as compared to the electrons, the positive ions contribute little to the discharge current. The discharge is essentially electronic, of a difiuse, luminous type and the current is approximately equal to the cathode emission.

The proximity of the virtual anode to the cathode permits the establishment of a potential gradient high enough to achieve saturation current, limited by cathode emission rather than space charge, at a much lower total voltage than would be required in a gas-free discharge path between the same cathode and the treated material. On

the other hand, the voltages that are employed are much greater than can be maintained across an are discharge, and the currents (for equal power dissipated) are correspondingly less. The use of saturated thermionicemission discharges of this diffuse, luminous character therefore permits high-power electric heating without the employment of excessively high voltages or large currents. Moreover, the high vacuum permits greater purification by evaporation of impurities from the melted material than does conventional arc melting, and the discharge is diifused over the entire surface of the melt, instead of being localized as in a hard core arc. Most of the electric power supplied to the discharge is utilized in accelerating the primary electrons to high velocities. The gaseous medium is sufficiently rarefied that few of the primary electrons experience collisions en route, so that their kinetic energy is mostly expended in bombarding the anode, whereby the anode surface is heated with good efliciency and uniformity.

One of the important uses for discharges such as that just described is for high-vacuum melting and casting of metals having a high melting point, a high chemical activity, or both, whose original reduction from their ores leaves them in either powder or sponge form. Examples of such materials are tungsten, titanium, and columbium, among many others. Rods and ingots can also be melted and recast. The raw materials usually contain impurities which can greatly affect the physical characteristics of the metal as finally produced. Many of the impurities commonly present in such materials have vapor pressures higher than that of the desired material at its melting point, so that high-vacuum melting can result in a high degree of purification of the material treated. Successive remeltings and recastings of the material in a high vacuum may be employed for greater purification. The very low gas pressure at which the present process is carried out facilitates the evaporation of the impurities, which are removed by condensation on cooler surfaces and by the vacuum pumps. A greater degree of purification can therefore be accomplished, or an equal degree in fewer remeltings, than is possible under the much higher gas pressures that are necessary to sustain the arcs employed in ordinary arc-melting, even when such melting is possible.

Such diffuse, luminous discharges as those described are only metastable at high power levels. If the highly conductive plasma extends itself too near the cathode structure--focusing electrodes, supports, or the cathode itselfand the voltage gradient near the cathode becomes excessive, a localized discharge of much lower resistance shunts the desired diffuse discharge, and if it persists, the localized discharge soon degenerates into a self-sustaining are that restricts the discharge voltage to a relatively low value until the arc is extinguished. Breakdown of this kind may occur as the result of bursts of gaseous matter released from the melt. A different cause of breakdown that becomes increasingly likely in higher-power operation, where the anode current exceeds 2 or 3 amperes at several thousand volts, is this: the nominally cold portions of the cathode structure (such as focusing shields) may in fact become quite hot, and this reduces their work function and renders electron emission easier. They may, however, be cool enough to condense vapors emanating from the melt, and the condensate contaminates their surface. Areas of such contamination on the cathode structure may further reduce the work function where they occur. Particularly if this is in a region of high potential gradient, considerable electron emission may occur from the nominally cold, contaminated surface, and initiate an undesired, low-resistance, local discharge.

Many of these low-resistance discharges are self-quenching. The discharges themselves quickly clean the cathode surface and may restore its emission to normal value, whereupon the localized concentration of current stops. Relatively stable operation can continue, even with a more-or-less continual succession of such short-duration, minor arcs or discharges. If, however, a localized discharge of this kind persists for more than about a second,

it can excite copious electron emission from even a clean cathode surface, or from adjacent, not-so-clean surfaces, which builds up the discharge progressively until a hardcore, self-sustaining arc is formed through an ionized path 'kept supplied by vaporization of the cathode material.

- ations in the gas pressure and density within the discharge.

An increase in gas density tends to increase the supply of ions, and the plasma expands toward the cathode. If the applied high voltage remains constant, expansion of the plasma tends to increase the voltage gradient at the cathode, which tends to increase the emission current, which tends to further increase the ion supply. Hence, breakdown is inevitable unless the above sequence of events is broken. On the other hand, a mere reduction of voltage proportional to decreases in the resistance of -the discharge path results in a fluctuating power dissipation at the anode, further fluctuations, with a time delay, in the evolution of gas, and a high probability, at high power levels, for the development of uncontrollable oscillations leading to breakdown.

The present invention provides a method and apparatus for so controlling diffuse, luminous discharges of the type described as to maintain them at maximum efficiency,

which in practice commonly means near instability. More specifically, major objects of the invention are to provide a method of control that will maintain both the voltage across a diffuse, luminous discharge and the power dissipated by it constant to within a narrow range of values, that will normally extinguish minor, localized discharges before they can develop into self-sustaining arcs, that will promptly extinguish any arcs that do form and upon their extinction immediately re-cstablish the desired, diffuse,

luminous type of discharge, and that will prevent the occurrence of dangerous excesses in either current or voltage if an arcing fault does occur. A further object of the invention is to provide apparatus whereby control can be exercised manually until optimum operating parameters have been determined for the treatment of a specific material and by which control can thereupon be transferred to automatic equipment for maintaining optimum operation.

In accordance with the present invention, operation is initiated by supplying sufficient electrical power directly from a power supply to a thermionic cathode to heat it to a temperature at which it will thermionically emit sufficient electrons to carry the current corresponding to the power desired in the main discharge at the desired operating voltage. With the desired emission established, voltage is supplied to the cathode-anode circuit to establish the discharge. Thereafter the current is maintained substantially constant at the established value; initially, the voltage across the discharge path will be higher than the normal operating voltage, but it will fall gradually as the material treated heats and by the evolution of gaseous matter and secondary electron emission establishes a zone of ionization. As this occurs, ionic bombardment of the cathode and other processes, particularly changes in the electric field distribution, tend to increase the electron emission; hence, in accordance with the invention, the

power supplied to heat the cathode directly from the power supply is reduced as a non-proportional function of the voltage between the cathode and anode, which controls the resistance of the discharge path for maintaining a substantially constant average applied high voltage. As the process proceeds and equilibrium conditions are established, the directly supplied, cathode-heating power is continuously varied as a non-proportional function of the discharge voltage to maintain the power and voltage of the main discharge both within their desired operating ranges. More specifically, the emission current is regulated at the power supply to prevent substantial fluctuations in the discharge current, and the cathode temperature is separately regulated to control the voltage gradient at the cathode for maintaining the average resistance of the discharge path and the average power dissipated at the anode substantially constant.

Any cathode structure has a certain thermal capacity, and there is a consequent time lag between changes in the power supplied to it, both directly from the power supply and indirectly from the discharge, and changes in the emission current. Generally, a decrease in the resistance of the discharge path, caused, for example, by the sudden release of a burst of gas from the melt, is countered by a reduction in the heating current supplied to the cathode, for the purpose of reducing the cathode temperature to increase the voltage gradient at the cathode. Hence, when an arc occurs, the cathode heating current might be reduced essentially to zero, and if the arc persists long enough the cathode may cool below the minimum emission temperature. Then, when the arc breaks the resistance of the discharge path will be very high, the

. applied high voltage will rise to an excessive value and a new are will be struck. According to another feature of this invention, such oscillatory conditions are avoided by a controlled supply of heating current to the cathode during arcing.

It will be seen that the operations thus described can be controlled manually, either in whole or in part; they have, in fact, been so controlled, and normally are so controlled in establishing optimum conditions for material of initially unknown quality or characteristics. Continuous manual control is, however, expensive, and becomes increasingly difficult with increase in size and capacity of the equipment to be controlled. The present invention therefore contemplates apparatus for performing automatically each of the steps described above.

The detailed description of the invention which follows is illustrated by the accompanying drawings wherein:

FIG. 1 is a diagram, partly schematic and partly in block form, of equipment embodying this invention;

FIG. 2 is a partial circuit diagram illustrating in more detail certain equipment symbolized in block form in FIG. 1;

FIG. 3 illustrates the general relation between emission current and voltage gradient at a hot cathode;

FIG. 4 illustrates the general relation to be established between cathode or filament current and applied high voltage for stable operation in the treatment of different materials;

FIG. 5 illustrates an approximate voltage distribution etween the cathode and anode.

FIG. 1 is a greatly simplified diagram, partly schematic and partly in block form, illustrating the elements of a control system for a high-vacuum, electronbombardment furnace, in accordance with the present invention. In

. this diagram the furnace is symbolized by a vacuum chamber 1, which is evacuated to an absolute pressure in the order or" one micron of mercury or less through a duct 3 by suitable pumps 5.

The discharge within the furnace takes place from a thermionically-emissive cathode 7 to an anode 9, which may be a molten pool of the material treated contained in a conductive crucible it and electrically grounded through the metal walls of the vacuum tank. Electrons emitted by the cathode are accelerated to high velocities by the cathode-anode voltage, and bombard and heat the molten surface of the material within crucible 10. Gaseous matter evolved from the melt becomes ionized and forms a plasma (a highly conductive ionized body of essentially neutral charge) extending outward from the melt. The principal voltage drop occurs between the cathode and the plasma; and the chief object of the invention is to control the discharge so as to keep the resistivity of this high-voltage region sufficiently high, and to prevent arcing, and other forms of breakdown, while operating at high power levels. In the drawing, focusing electrodes, heat shields, crucible-cooling means, provisions for continually supplying and withdrawing the material treated, and the like, with which the present invention is not directly concerned, have been omitted for simplicity and clarity.

The cathode 7, in a preferred form of the device, is a filament in the form of a single loop of tungsten wire or rod. Current passed through the loop heats the cathode to electron-emitting temperature, and leads for supplying this current are brought in through the sides of the chamber by insulating bushings, symbolized at 11. Cathode-heating current is supplied through a filament transformer 13 which, in the usual arrangement, derives its power from a 60-cycle commercial source. Heating current is controlled by means of a saturable reactor 15, controlled as will be described hereinafter.

High voltage between the cathode 7 and anode 9 is provided by a constant-current D.-C. supply, whereby the applied D.-C. voltage is proportional to the resistance of the discharge path between the cathode and the anode. In the embodiment of the invention illustrated, the power used is derived from a three-phase, 60-cycle commercial source. Power from that source is first supplied to a conventional, three-pl1ase constant-current network, preferably of the type known as a Steinmetz constant-current network, indicated generally by the reference character 17. This network comprises three delta-connected legs, each including an inductor 19 in series with a condenser 21 tuned to series resonance at the supply frequency. Schematically the inductors 19 and condensers 21 are shown as variable, for the sake of simplicity; in practice the condensers preferably take the form of tapped condenser banks and the inductors are also tapped so that the legs can be tuned and the output current adjusted by selection and interconnection of the proper taps.

The three-phase input leads 23a, 23b and 230, are connected to the apices of the delta network, so that the input phase vector rotates into the inductor of each leg firsti.e., counterclockwise in network 17 as illustrated in FIG. 1. The output leads 25a, 25b and 250 connect to the junction between the inductor and the condenser of each leg. As is well known, when the circuit illustrated is properly tuned, the voltage developed across the output leads is very nearly proportional to the effective impedances connected across these leads, with the corollary that the current supplied to the output circuit is very nearly constant. For present purposes, no substantial error is involved by considering it to be a constant, and it Will be so treated in what follows.

Output leads 25 of the constant-current network connect to the delta-connected, primary 27 of a three-phase, step-up transformer. The secondary 29 of this transformer is star connected, in the example illustrated. The secondary is connected to a rectifier bank comprising, for example, six mercury-vapor rectifiers, collectively designated by the reference character 31. These rectifiers are connected in known manner to lead 35.

Various other types of constant-current supply are available, and the invention is not limited to the use of any particular type. The arrangement shown is preferred because of its simplicity and high efiiciency. The current can be reduced, when so desired, by manually detuning the network.

Because of the constant-current, variable voltage characteristics of the network 17, it should be obvious that if the output circuit of the rectifier bank is opened the voltage delivered to it can rise to very high values; in one such network, supplied by a 440 volt input, the output voltage on open circuit was computed at 13 kv., with dangerously high circulating currents in the network. As asafety measure if is therefore preferable to provide horn gaps 33 across each phase of the secondary winding 29, adjusted to break down at some convenient, predetermined value of output voltage. In one apparatus embodying this invention these gaps are set to break down at 7.5 kv.; the rectifier network, therefore, supplies a DC. output with substantially. constant current at voltages in the range between substantially zero and 10 kv.

It will be realized that no constant-current device, can provide absolute constancy of current through an unlimited range of impedances. Practical constant-current devices, as the term is used in the art, are characterized by a dynamic impedance,

dE Z17 relatively much higher than their effective impedance within their operating range. Thus a constant-current source, such as the network 17 as viewed from the discharge through the rectifier bank, will limit the current through the discharge to a definite maximum value even if the cathode and anode are shorted, so that the impedance of the gap and the voltage across it approach zero, and will maintain the current within a few percent of that maximum even if the effective impedance of the gap rises to such a value that the potential required rises to several thousand volts.

One side of the rectifier bank is grounded. The other side connects through lead 35 to the secondary of the filament transformer 13 and thence to the cathode 7. For supervisory purposes, and as an aid to manual control, an ammeter 37 is preferably provided in the ground lead of the rectifier bank to indicate the discharge current. A voltmeter 39, in series with an external multiplier resistor 40, connects from lead 35 to ground for like purposes.

Also connected from lead 35 to ground is a high resistance voltage divider consisting of resistor 41 and potentiometer 42 in series. Lead 53, connected to the circuit junction between elements 41 and 42, picks off of the voltage divider a small voltage, generally about -200 volts, proportional to the voltage across the discharge in the furnace, and feeds it to a variable-gain D.C. amplifier 47, which is adjustably biased, as hereinafter explained. Output current from the amplifier 47 is supplied to a winding 49 of the saturable-core reactor 15, to supply thereto a current directly related to the voltage across the discharge path within the furnace. Increase in this current, increasing the saturation of the core, decreases the effective reactance of reactor 15 and thereby increases the electrical energy supplied to heat the cathode 7 and thus increases its temperature. A decrease in voltage has, of course, the opposite effect.

The conditions existing within the discharge path are not only complex but change from moment to moment owing, in part at least, to variations in volume and position of the ionic plasma. Some understanding of the nature of the control exercised by the cathode temperature can be derived from a consideration of FIGS. 3 through 5.

FIG. 5 shows the approximate voltage distribution between the cathode and the anode. Gaseous matter released from the molten anode becomes ionized and forms a low-resistance, ionic plasma adjacent to the anode. Because of its high electrical conductivity, there is little voltage drop across this plasma, and its outer surface,

s e sees which is at nearly the same potential as the molten anode, forms a virtual anode which attracts electrons emitted by the cathode. Between the cathode and the plasma, the resistivity of the gaseous medium is relatively high, and it is here that most of the voltage drop across the discharge appears. In practice, the plasma expands and contracts, and if the discharge voltage, and hence power, is to be kept constant, it is evident that the voltage gradient near the cathode must change accordingly.

The curves illustrated in FIG. 3 are somewhat related to the familiar current-voltage curves of a diode, carried into the current-saturation range, and are necessarily only rough approximations in view of the very complex nature of the phenomena under consideration. The currents are plotted as ordinates; because, however, the size and position of the ionic plasma, constituting a virtual anode, is variable, and in varying changes the configuration as well as the actual length of the discharge path through the relatively high-resistance region, the abscissas represent the voltage gradient in the immediate vicinity of the cathode and not the total voltage across the discharge. The gradient varies as a direct, non-linear function of the total voltage and an inverse, non-linear function of the length of the high-resistance zone between the cathode and the plasma.

Subject to this understanding of the meeting of the curves, the cathode emission current varies with the voltage gradient at the cathode in approximately the manner shown. In the region of space-charge limitation, the gradient at the cathode is substantially zero, or even slightly negative. As the current is increased, the point commonly called saturation is reached, where nearly all of the electrons emitted by the cathode are drawn over to the anode. By continuing to increase the applied voltage, or to decrease the spacing between the cathode and the virtual anode, a positive voltage gradient can be obtained fairly close to the cathode, and thereby, due to many, complexly interrelated factors, not requiring discussion here, a small but significant increase in current over the saturation value can be obtained. According to the present invention, the cathode is operated in this beyondsaturation region. Of course, if this is pushed too far and the voltage gradient becomes too large, breakdown and arcing will occur.

Emission current also varies as a function of cathode temperature. In FIG. 3, curve A applies to one cathode temperature, while curve B applies to another, somewhat higher cathode temperature. If the cathode becomes too cool, no substantial emission occurs, in the high vacuum under consideration, until the voltage gradient becomes so high as to cause almost immediate breakdown. Hence, the cathode must be kept above the minimum temperature for thermionic emission, which depends on the cathode material.

The dotted line C of FIG. 3 indicates the current supplied by the constant-current network 1'7; it has a slightly negative slope, indicating the slight decrease in current as the effective impedance across the discharge gap increases from zero to a relatively high value. In the process under consideration, ionic currents are very small in comparison to the electronic currents; therefore, the supply current and the emission current must be approximately equal. Hence, if the cathode temperature corresponds to curve A, the operating point is the intersection of curves A and C, and the voltage-gradient will be that corresponding to the abscissa of point X. An increase of cathode temperature to that corresponding to curve B will drop the voltage-gradient cordinate to point Y. Hence, other factors remaining the same, the cathode temperature determines the voltage gradient. In practice, other factors do not remain the same for long, and the cathode temperature is varied to control and stabilize the discharge. The total cathode-anode voltage automatically adjusts itself, through the constant-current supply to the value determined by the gradient established and the discharge geometry. The relatively large changes in voltage with small changes in cathode temperature give a powerful negative feedback with which to hold the average voltage substantially constant.

Within the above-saturation range, the voltage gradient near the cathode approaches the total voltage applied across the discharge gap divided by the distance between the cathode and the virtual anode. It will be apparent from the curves of 3 that a change in cathode temperature can compensate for a change in position of the virtual anode to maintain the voltage (and therefore the power) constant, since the current is substantially constant by postulate.

The position of the virtual anode, however, depends (among other factors) on the rate at which gaseous matter is evolved from the anode, either from evaporation of the treated material or liberation of impurities. It is therefore a function of the power dissipated in the discharge (which mostly goes into electron bombardment of the anode) among several other factors, including the structure of the furnace and the composition of the raw iaterial, its melting point, vapor pressure, and purity.

The nominal current and voltage ratings are design parameters of the particular furnace to be controlled. To establish stable operating conditions for a specific melt, the cathode temperature may first be set to give a higher emission than required to carry the value of current delivered by the constant-current network 17. Initially, the discharge will be space-charge limited, the resistance of the discharge path will be high, and the oathode-anode voltage will be correspondingly high. As the bombarded and heated anode releases gaseous matter, ions form which lower the resistance of the discharge path, and the voltage drops. The cathode-heating current can then be gradually reduced to keep the voltage and power at the desired values. Whenever the voltage (and hence the power) drops below the desired value, the cathode temperature should be lowered to increase the voltage gradient and the resistance of the discharge; conversely, whenever the voltage rises above the desired value, the cathode temperature should be raised. The operation can now be switched over to automatic control, which will maintain the voltage and power at substantially constant value.

With materials of known characteristics the operating points can be pre-set and transfer to automatic control effected promptly. To achieve stable operation at the highest possible power level, with materials of unknown characteristics, particularly as to their impurity content, ordinarily requires further adjustment, because of the effect of the material treated on the discharge itself.

The quantities that aifect the discharge areso interrelated that operating parameters cannot be set arbitrarily. The density and pressure of gaseous matter at the anode depends on the temperature of the melt, the material melted, and particularly on its purity. The rate at which the cathode loses energy by radiation also depends, in part, on the anode temperature, and this, in turn, affects the direct heating energy that must be supplied to it to maintain its temperature. Too much power in the discharge heats the melt too fast, speeds up the evolution of gaseous matter, increases the volume of the ionic plasma, and decreases the spacing between the ionic plasma and the cathode, whereby the discharge becomes unstable and forms a low voltage arc. Too little power has opposite effects, and the ion density may become so low that electronic space charge limits the current, and the voltage rises to excessive values. Thus, for each different material there is a different set of operating parameters for achieving the type of operation desired. These parameters must be determined experimentally.

Furthermore, the characteristics of the melt, the rate of evolution of gaseous matter, and the size of the ionic plasma may change during the course of a melting operation. To maintain the desired power level, at constant current, the average resistance of the discharge path must be kept substantially constant. According to this invention, the resistance of the discharge is regulated by controlling the cathode temperature responsive to the cathode-anode voltage. When the voltage drops, the heating current to the cathode is reduced; and when the voltage rises, the heating current is increased. However, the heating current is not proportional to the voltage; but may be proportional to the algebraic sum of the voltage and a negative constant of approximately the same magnitude. In mathematical terms, if I represents the cathode heating or filament current, and V represents the voltage between the cathode and the melt, stable operation may be achieved by controlling the filament current so that where S and K are experimentally determined constants which have different values for difierent materials, but have never been found to be zero. In general, S is usually only slightly smaller--say, lessthan the average value of V.

It might be supposed that the more rapidly the cathode heating current responded to changes in discharge voltage the more stable would be the control. This is not the case in fact. The thermal capacity of the cathode introduces delays in its response to changes in heating current, which are tantamount to a phase delay around the feedback loop, which varies with anode temperature and the other factors dependent upon it, so that too rapid a response to voltage changes can result in instability, with violent oscillatory changes in discharge voltage. Also, short-duration voltage fluctuations actually help to stabilize the discharge. For example, frequent, short-duration, localized discharges may develop between the ionic plasma and the cathode or other parts of the apparatus. These localized discharges have relatively high current densities and low resistances; they cause a sudden drop in the cathode-anode voltage, which helps to extinguish the local discharge before it can develop into a self-sustaining arc. Hence, it is not desired to eliminate all voltage fluctuations; it is desired to keep the average voltage approximately constant.

In addition to the above factors, there is a threshold temperature below which the cathode will not emit any considerable number of electrons, but secondary effects may be sufficient to maintain the cathode at full emission even though the heating current supplied to it may be insufiicient of itself to raise it above the threshold temperature; the resistance of the cathode varies with temperature, and the heating current supplied to the cathode may vary non-linearly with the control current supplied to the saturable reactor 15.

In practice all of these inter-related factors can be resolved by the adjustment of two operating parameters: the setting of a bias or off-set voltage in amplifier 47, and the adjustment of the amplifier gain. These adjustments will be discussed in more detail in the description of the circuits illustrated in FIG. 2.

The effects of these adjustments are illustrated in FIG. 4, wherein filament current is plotted against cathodeanode voltage. Curves D and E of this figure show typical characteristics for stable operation in treating two different materials. The important facts to observe about these curves are their different slopes and their different intercepts on the zero current axis; these intercepts are never at the origin if stable operation is to be maintained. Stated otherwise, neither the cathode heating power, current nor voltage is directly proportional to the discharge voltage.

Equipment whereby all of the necessary adjustments may initially be made manually and then be transferred to automatic control is illustrated in FIG. 2. It is more convenient, however, to describe first the automatic control equipment, as the manual control elements are, in

'10 one sense, apparatus whereby the automatic control is calibrated.

Such parts of the apparatus illustrated in FIG. 1 as are necessary to the complete description of the second figure are designated by the same reference characters as in FIG. 1. The equipment comprised within the block 47 of FIG. 1 is shown enclosed within broken lines 47 of FIG. 2; similarly the equipment within block 51 of FIG. 1 is enclosed within broken lines 51 in FIG. 2.

The resistors 41 and 42, forming a voltage divider for providing a convenient voltage of about-200 volts proportional to the much higher cathode-anode voltage, are shown at the extreme right of FIG. 2, connected between ground and the lead 35 that connects the high-voltage power supply to the center of the secondary of filament transformer 13 which feeds the cathode 7. Lead 53, extending from the circuit junction between resistors 41 and 42, connects through one pair of contacts 55 of a ganged, automatic-manual changeover switch, to one end of a p0 tentiometer 43. The lower end of this potentiometer connects to the moving contact of a second potentiometer 57, one end of which goes to ground and the other to a source of negative potential, preferably a negative tap on a conventional power-pack.

The setting of potentiometer 57 determines the bias setting of the amplifier, and thus determines the approximate average voltage between cathode 7 and the melt during automatic operation. This bias may be set at any value between ground potential and 275 volts; for the particular apparatus illustrated it will usually be somewhere in the neighborhood of volts, so that the grid of the first amplifier tube is about ten volts more negative than its cathode with normal voltage across the discharge in the furnace and lead 53 at about 2OO volts. In plotting cathode-heating current as abscissas against discharge voltage as ordinates, as shown in FIG. 4, the setting of potentiometer 57 effectively determines the average voltage across the discharge during automatically controlled operation. On this same plot the setting of the potentiometer contact 45 determines the slope of the curve, i.e., the rate at which the filament current is increased with increasing voltage across the discharge, during automatic operation.

The contact of potentiometer 57 connects directly to one cathode 53 of a dual triode 59, while contact 45 connects directly to the grid 60 for controlling the current in this triode. The. anode of the same tube section connects through a load resistor 61 to the adjustable tap of a potentiometer 63 connected from a +250 volt tap on the power supply to ground. This arrangement makes it possible to adjust the average anode voltage of the vacuum tubes, which is usually done during factory calibration of the amplifier.

The drop across resistor 61 is applied directly to the grid 69 of the second section of tube 5% through the usual protective resistor 65. The cathode 58 of the second section of the tube is connected back to potentiometer 57 through a cathode resistor 67, whereby this section of the tube 59 acts as a cathode follower.

The voltage developed between the cathode 58 and ground is applied to the four control grids of two dual tubes 65 in parallel. The cathodes of these tubes are grounded directly; their anodes connect, also in parallel, through small protective resistors 71 to the control winding 49 of the saturable reactor 15. The current controlled by these tubes is supplied from a +200 volt tap on the same power supply as is used to provide the other operating voltages for the amplifier circuit.

Following through the connections described, it will be seen that increased voltage drop across the discharge path within the furnace drives the grid of the first section of tube 59 more negative, thus decreases the drop across resistor 61 and drives the second grid toward positive. The second tube section being connected as a cathode follower, it in turn drives the grids of all of the tubes 69 toward aorases positive, thus increases the current through these tubes and through the control winding 4-9 of the reactor, and increases the cathode heating current, which tends to raise the cathode temperature and to lower the resistance of the discharge path. At constant current, a drop in discharge resistance lowers the voltage, and thus a negative feedback stabilizing action is achieved which tends to hold the discharge voltage and power substantially constant, on the average. In a sense, adjustment of tap 45 varies the gain of the feedback loop, which because of the substantial time delays and other complex factors involved, must be neither too high nor too low if stable operation is to be maintained.

The voltage at the cathode 58 of the output section of tube 59 is also applied through a second set of contacts 55 on the automatic-manual changeover switch to a lead 73, connected through a variable resistor 75 to the cathode of a thyratron 77 and also, through a condenser 79, to ground. The control electrode of the thyratron is connected through a current-limiting resistor 81, to the movable tap of a potentiometer 33, incorporated in a voltage- -divider-string connected between the -275 volt tap of the power pack to ground. The anode of thyratron 77 is connected directly to ground.

In practice potentiometer 33 can adjust the voltage on the'control electrode of thyratron 77 through a range of 50 or 60 volts, from approximately 110 volts negative to somewhere in the neighborhood of 160 volts negative. In normal operation, the positive voltage drop across cathode resistor 67 and the negative voltage of approximately l90 volts from potentiometer 57 add algebraically to make the cathode of thyratron 77 sufficiently positive relative to its control grid that the thyratron remains nonconductive. However, if the voltage across the discharge in the furnace drops, the current through resistor 67 also drops, and at some pre-set minimum value of this current, after a time delay determined by the values of capacitor 79 and resistor 75, thyratron 77 strikes and effectively connects resistor 75 between ground and resistor 67. This forms a voltage divider which holds cathode 53 and the grids of tubes 69 at a sufliciently small negative potential for the conduction of considerable current through tubes '69 and control winding 49, whereby heating current is restored to filament 7 before it can cool to less than minimum emission temperature.

Striking of the thyratron 77 also reduces the resistance effective across the condenser 79 substantially to zero, discharges tlie condenser, brings cathode and anode of the thyratron substantially to the same potential, and thus breaks the discharge in the thyratron. If the arc in the furnace has not broken by this time, the condenser recharges through resistor 75, the thyratron strikes again, and continues to make and break the circuit across condenser 79 as long as arcing in the furnace continues. Tube 77 therefore acts as an approximately sawtooth oscillator. The frequency and breakdown point can be varied, respectively, by shifting the contacts on the resistor 75 and the potentiometer 83. Between these two adjustments the time during which reactor is saturated, and hence its average impedance, can be adjusted so as to pass enough current to maintain the temperature of cathode 7 in the emitting range. Furthermore, it should be noted that there is a time delay, adjustable through variable resistor 75, between a drop of voltage across the discharge in the furnace and the firing of thyratron 77. Hence, the thyratron circuit does not operate responsive to localized discharges of such short duration that its action is not needed.

The core of the saturable reactor 15 is never fully saturated while thyratron 77 is oscillating as described above, and control-winding 49 has a considerable inductance; hence the current passed by it is an inverse function of frequency. Therefore the degree of saturation of the reactor core can be controlled by adjusting the frequency of the thyratron oscillations by adjusting resistor 75. This permits the heating power supplied to the cathode under such conditions to be set at a point which will bring it up to any desired temperature within the operating range, so that when the are in the furnace does break the shoetive resistance of the cathode-anode gap will be reasonably close to the normal operating value and the voltage across it will not rise to so high, a recovery Value that it would either re-establish the arc in the furnace or cause breakdown at the horn gaps in the power supply and possibly require that the main power supply be cut oil to re-establish operating conditions.

If a self-sustaining arc is established which carries the normal operating current of the network 17, it is clear that additional means must be invoked to break it. This can, of course, be done by opening the circuits to the constant-current network 17. It is preferred, however, to avoid undesirable transients following re-closure of the circuit. It is desirable, too, that normal operating conditions be reestablished as soon as possible following the breaking of an are. One way of accomplishing this is illustrated at the right of FIG. 2.

A potentiometer tap is taken off from the resistor 42 at the low potential end of the voltage divider string, which connects to the control grid of a thyratron 87. The cathode of this thyratron connects to ground through a variable resistor 88; its anode connects to the winding of a relay 89 and thence, through a condenser 91, to the +250 volt tap of the amplifier power supply. Condenser 91 is bridged by a high resistance 92, of a value such that the time constant of the condenser-resistor combination is in the order of one second.

Potentiometer contact 85 is set to a point that will hold thyratron 87 nonconductive until the voltage across the cathode-anode gap of the furnace drops to the low value indicative of an arc. At this point thyratron 87 fires and charges condenser 91 through the winding of relay 89, tube 87, and resistor 83, in series. The last-mentioned resistance is much smaller than resistor 92, and it is adjusted so that the time constant of the series circuit which includes the relay winding and condenser 91 is of the order of one-sixtieth second. Upon firing of the tube 87, relay 89 closes and actuates the magnetic contactor 93. The latter is supplied by the main AC. power source to close the contacts and short each leg of the constant-current network by closing the circuit interconnecting each pair or" the output leads 25 25 and 25 which are shown fragmentarily in the figure. Current flows in the coil of relay 89 only until condenser 91 is charged substantially to the 250 volt supply voltage, at which time the discharge through tube 87 breaks and relay 89 reopens. This, in turn, permits magnetic contactor 93 to de-energize and contacts 95 to open, breaking the short circuits across the legs of the constant-current network.

The contacts 95 carry only the current normally supplied to transformer primary 27, because of the characteristics of the constant-current network, but the voltage across the rectifiers 31 immediately drops substantially to zero and causes the are within the furnace to break. This usually happens within a half-cycle of the 60-cycle input power. By adjusting the two time-constants associated with condenser 91, the time during which relay 89 re mains closed can be adjusted so that it is long enough to insure the breaking of the are within the furnace, but no longer. The capacity of condenser 91 must be large enough to store sufiicient energy to hold relay 89 closed for the required interval. This will, of course, depend on the sensitivity of the relay. The values of resistors 88 and 92 are chosen accordingly.

it will, of course, be recognized that electronic switches, such as ignitrons or thyratrons, can be substituted for the electro-mechanical relays and contactors here illustrated, and that the shorting connections may be made across the secondary 29 of the transformer, or between lead 35 and ground.

To switch the apparatus to manual control the ganged switches 51 and 55 are thrown to the opposite position from that shown in the drawing. This disconnects potentiometer 43 from the amplifier input lead 53, and connects it instead to a manually-operated, variable resistor 97, which connects to the 275 volt tap on the source. The cathode of tube 59 remains connected to the poteniometer 57, however, so that current liows from the negative source through resistor 97 and potentiometer 43 back to potentiometer 57 and thence to ground. The heating current supplied to filament 7 can now be controlled manually by adjusting the position of tap 45 on potentiometer 43.

Resistor 97 is preferably calibrated in terms of the gap voltage indicated by voltmeter 39, so that the current through potentiometer 43 at a given setting is the same as at the indicated voltage when on automatic control. The current through the cathode filament 7 can be read on an ammeter 99 in the primary circuit of transformer 13, and when proper operating conditions have been established this current can be matched by the automatic setting. Obviously an entirely separate control could be used for regulating the cathode heating current, but although such separate control mechanism would be much simpler per se than operating through the amplifier 47 it would actually add to the complexity of the system, and make transfer from manual to automatic operation more difiicult.

As has been stated, the biasing point for tube 59, which determines the average voltage across the discharge in the furnace during automatic control, is set by potentiometer 57 and will usually difier least of all of the operating parameters as between different materials treated. This potentiometer therefore seldom needs readjustment. Much more critical is the slope of the filament current vs. discharge voltage curve. This is set by means of the movable tap 45 on potentiometer 43. In the present case this is done by means of a small, reversible, electric motor 1M, which may be operated to adjust the position of contact 45 by operating one or the other of push buttons 103.

It will be seen that the operation of contact 55 to the manual position disconnects the circuits of tube 77 from direct connection with resistor 67, and therefore disables the equipment for overriding the automatic cathode-current control when arcing occurs. Connection of the circuits of tube 77 can be re-established by operating push button 195. The preferred procedure is to set resistor 97 to correspond to a value of gap voltage that would indicate arcing. Push button 165 is then depressed and potentiometer 33 adjusted until tube 77 fires. This in indicated by a sudden increase in the reading of ammeter 99, from say 2 or 2.5 amperes to 5 or 6 amperes. Adjustmeut of resistor '75 then establishes the cathode current at the desired value.

Similarly, it may be desirable in establishing working conditions to open switch 107 and disable the shorting circuit 51 until a stable operation is established. If arcing does occur, the arc discharge can be broken by simply closing switch 167.

In practice it has been found that one of the major advantages provided by this invention lies in the fact that the various interrelated factors that maintain stability of the discharge can be adjusted manually during experimental operations until an optimum operating point is found for each specific parameter, and the control of the various operating parameters can be transferred to the automatic control equipment, one-by-one, as these points are determined. Furthermore, because of the order in which the steps are taken, the danger of destructive voltages or currents that would normally be inherent in supplying large amounts of power to a load of unpredictably varying resistance from a constant current source, or from a constant voltage source as in common prior practice, is avoided.

Through the procedure described the power dissipated 1d at the molten surface of the material treated can be maintained constant, to within +5 percent or less, over long periods of operation where the materials melted are reasonably pure or are of constant composition. This constancy is to be expected where the discharge is used to remelt and further purify metals that have previously been melted and cast in a vacuum.

Where the material of the melt is extremely gassy, as, for example, some of the metallic sponges as supplied by the primary producers of such materials, the term maintaining the power in the discharge constant is applicable only in connection with average, rather than instantaneous, power. It is frequently a characteristic of such materials that they contain gas inclusions which are released into the discharge in sudden, violent bursts. When such a burst occurs the volume of gaseous material that is released may be so great as to raise the pressure within the vacuum chamber to a point where the entire volume is filled momentarily with a luminous discharge, even though the pumps used to evacuate the chamber may have capacity suflicient to keep the pressure outside of the discharge path down to a fraction of a micron of mercury were the gas liberated at a constant rate.

Using conventional types of power supplies to treat such gassy materials with metastable discharges of the type here considered is impossible. Once a localized discharge is established with such a system it is followed .up by a rush of current that would quickly establish a self-sustaining arc and throw the apparatus out of operation until circuit breakers could be reclosed and the whole operation started afresh.

With the current in the diffuse discharge limited as herein described, no such current rush can occur and in the usual case the pumps quickly scavenge the released gas and any momentary, localized discharge breaks of itself, but during the persistence of the low-resistance discharge the voltage across it will drop, from say five to six thousand volts to something of the order of a hundred volts or even less. Less violent bursts of gas-emission cause less violent but still substantial fluctuations in discharge resistance.

The low-resistance discharge following a burst of gas emission may persist for time intervals varying from a small fraction of a second to several seconds, and in spite of the thermal capacity of the cathode, its temperature in comparison to its immediate surroundings is so high that it is quite possible for it to drop below minimum emission temperature in less than a second. if any of the usual types of constant-current supply were used and the discharge broken, with the cathode either non-emit ting or emitting fewer electrons than required to carry the constant current, the result would be a voltage of a value that could be even more destructive than the shortcircuit currents that would develop were the supply from the more usual prior-art constant-voltage system. In the practice of the present invention this is prevented by overriding the negative feedback type of control which renders the discharge operationally stable except when more than normal volumes of gas are liberated. Hence where the material treated is of such nature that continuous stability within very narrow limits becomes impossible, the present invention operates to prevent instantaneous interruptions of the desired process from developing into complete breakdown and permits immediate re-establishment of substantially the desired operating conditions, and by so doing permits large-scale commercial melting and casting operations at higher power levels than were practicable heretofore.

In treating gassy materials, where the most probable type disturbance to the discharge is from gas bursts, it may be desirable to disable the shorting circuit 51 by opening switch M7, and to operate this latter equipment manually and only when arcing is not cleared by the operation of the pumps. It may be added that is upon operations on such gassy materials that the effective gain 15 of the amplifier controlling the filament current should be reduced to compensate for the extremely violent variations in discharge voltage that are characteristic with materials of this class.

The specific apparatus that has been illustrated offers a convenient means of practicing the method automatically and of transferring manual to automatic control and vice versa. t is not, however, intended that the scope of the invention be considered as limited by the specific apparatus illustrated, all intended limitations being specified in the claims.

What is claimed is as follows:

1. The method of controlling an electric discharge in a vacuum between a hot cathode and a bombardmentheated anode, which comprises supplying heating current to said cathode su'fficient to produce electron emission and maintain the discharge between said cathode and said anode, the electrons so emitted bombarding and heating the anode thereby causing the evolution of gaseous matter therefrom and producing an ionic plasma between the anode and the cathode, supplying a substantially constant direct current to the discharge between said cathode and said anode, and continually adjusting said heating current as an inverse function of the voltage across said cathode and anode and thereby maintaining a substantially constant average. voltage between said cathode and said anode.

2. The method set forth in claim 1, wherein the heating current is continually adjusted to a value proportional to the difference between the voltage across the discharge and a constant in the order of smaller than said voltage.

3. The method set forth in claim 1, which additionally comprises adjusting said heating current to a fixed value upon the occurrence of low-resistance discharge shunting the desired discharge.

4. The method set forth in claim 1, which additionally comprises interrupting said constant current upon the occurrence of an arc shunting the desired discharge.

5. in the operation of apparatus for heating materials in vacuo by bombardment with an electron discharge through a discharge path from an electrically heated thermionically emissive cathode to an anode structure comprising the material to be treated, said discharge being supplied from a substantially constant-current source, the method of controlling said discharge to maintain therein a metastable state wherein a portion only of said discharge path is occupied by a plasma or" ions which comprises the steps of supplying the electrical energy directly to heat said cathode to a temperature whereat its total electron emission is substantially equal to that required to carry the constant current supplied by said source, passing current from said source through said discharge path, and varying thhe electrical energy supplied directly to heat said cathode as an inverse function of the voltage cross said discharge path to maintain said voltage constant to within a limited range.

6. The method set forth in claim 5 additionally comprising the step of increasing the electrical energy supplied directly to heat said cathode to a fixed value when the voltage across said discharge path falls below said limited range.

7. The method set forth in claim 5 additionally comprising the step of increasing the electrical energy supplied directly to heat said cathode to a fixed value when the voltage across said discharge path falls to a value indicative of the degeneration of said discharge into an are.

8. The method of controlling an electrical discharge in vacuo from a thermionically emissive, electrically heated cathode to an anode to be heated by said discharge and which anode evolves gaseous matter when heated so as to keep the average voltage across said discharge constant to within relatively narrow limits at a substantially constant current therethrough while maintaining said discharge in a metastable state wherein a low-resistance ionized zone is established adjacent to said anode a high-resistance zone adjacent to said cathode, which comprises the steps of continuously evacuating the space between said cathode and anode, supplying electrical energy directly to heat said cathode to a temperature at least sufficient to cause emission of an electron current equal to said constant value, continuously passing a current of said constant value through said discharge thereby heating said ano e and causing the evolution of gaseous matter therefrom, adjusting the electrical energy supplied to heat said cathode to bring the voltage across said discharge within said limits while maintaining the electron current emitted from said cathode substantially at said constant value, and thereafter varying the electrical energy supplied directly to heat said cathode as an inverse function of the voltage across said discharge to maintain said voltage within said limits irrespective of other agencies heating said cathode.

9. The method set forth in claim 8, additionally comprising the steps of increasing the electrical energy supplied directly to heat said cathode to a fixed value suitiient of itself to produce an electron-emission current equal to said constant current and reducing the current in said discharge below said constant value during intervals when the voltage. across said discharge falls materially below said limits.

10. Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma, comprising a thermionically emissive, filamentary cathode, a bombardment-heated anode, a constanturrent D.C. power supply connected across said anode and cathode for supplying thereto a substantially constant direct current at a voltage substantially proportional to the resistance of the discharge, filament-current supply means for supplying electric power to heat said cathode to electron-emitting temperature, connections providing a first electric signal that varies with changes in the voltage across the discharge, means providing a bias in bucking relation to said first signal, amplifying means connected to amplify the difference between said first signal and said bias to provide a control signal, and control means responsive to said control signal for automatically adjusting the power supplied to heat said cathode, whereby the average voltage across the discharge is automatically kept substantially constant.

ll. Apparatus as in claim 10, wherein said control means is a saturable reactor having a variable-impedance winding connected between said filamenucurrent supply and said filamenatry cathode, and having a control winding connected to said amplifying means for receiving said control signal.

12. Apparatus as in claim 10, wherein said anode is grounded, the connections for providing said first signal comprise a voltage divider connected between said cathode and ground, the means providing a bias comprises a source of negative potential and a potentiometer connected between said source and ground, and said amplifying means has two inputs respectively connected to said voltage divider and said potentiometer.

13. Apparatus for controlling an electric discharge in vacuo, wherein the path of said discharge includes a lowresistance ionized zone and a high-resistance Zone, comprising a thermionically emissive cathode, an anode heated by said discharge, means connected across said anode and cathode for supplying thereto a substantially constant direct current at a voltage substantially proportional to the resistance of the discharge, cathode-heating means for supplying electric heating power to heat said cathode to electronemitting temperature, a saturable reactor connected to said cathode effectively in series with said cathode-heating means, means connected across said anode and cathode for deriving a voltage proportional to the voltage across said discharge, and a direct current amplifier connected to respond to the so-derived voltage and to provide to said saturable reactor a control current varying with the so-derived voltage from a maximum producing substantial saturation of said reactor to a minimum at cutoff of said amplifier.

14. Apparatus as in claim 13, additionally comprising means for varying the rate-of-change of said saturating current with variation of said control voltage.

15. Apparatus as in claim 13, additionally comprising adjustable biasing means for varying the value of said control voltage at which said amplifier cuts 011.

16. Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma, comprising a thermionically emissive, fila mentary cathode, a bombardment-heated anode, a constant-current DC. power supply connected between said cathode and said anode for supplying a substantially constant direct current to the discharge, filament-current supply means connected to supply heating current to said filamentary cathode, an amplifier connected to provide an electric control signal responsive to voltage changes between said cathode and said anode, control means responsive to said control signal for varying the cathodeheating curent to keep the average value of the cathodeanode voltage substantially constant, a relaxation oscillator including a thyratron, means normally biasing said thyratron beyond cutoflf to keep said oscillator out of operation, connections for overriding said biasing means to bring said oscillator into operation responsive to abnormal drops in the voltage between said cathode and said anode, and means for maintaining the supply of heating current to said filamentary cathode while said oscillator is in operation, whereby said cathode is kept at emitting temperature until normal operation is restored after arcing.

17. Apparatus for controlling an electric discharge in a vacuum wherein evolved gaseous matter may form an ionic plasma, comprising a thermionically emissive cathode, a bombardment-heated anode, a constant-current DC. power supply connected between said cathode and said anode for supplying a substantially constant direct current to the discharge, means for automatically controlling the temperature of said cathode to keep the average voltage between said cathode and said anode substantially constant, and means for automatically shorting said power supply upon a decrease of the voltage between said cathode and said anode to a low value relative to said average voltage.

References Cited in the file of this patent UNITED STATES PATENTS 2,159,767 Liebich May 23, 1939 2,310,286 Hansell Feb. 9, 1943 2,408,091 Olesen Sept. 24, 1946 2,850,676 Kan Sept. 2, 1958 

1. THE METHOD OF CONTROLLING AN ELECTRIC DISCHARGE IN A VACUUM BETWEEN A HOT CATHODE AND A BOMBARDMENTHEATED ANODE, WHICH COMPRISES SUPPLYING HEATING CURRENT TO SAID CATHODE SUFFICIENT TO PRODUCE ELECTRON EMISSION AND MAINTAIN THE DISCHARGE BETWEEN SAID CATHODE AND SAID ANODE, THE ELECTRONS SO EMITTED BOMBARDING AND HEATING THE ANODE THEREBY CAUSING THE EVOLUTION OF GASEOUS MATTER THEREFROM AND PRODUCING AN IONIC PLASMA BETWEEN THE ANODE AND THE CATHODE, SUPPLYING A SUBSTANTIALLY CONSTANT DIRECT CURRENT TO THE DISCHARGE BETWEEN SAID CATHODE AND SAID ANODE, AND CONTINUALLY ADJUSTING SAID HEATING CURRENT AS AN INVERSE FUNCTION OF THE VOLTAGE ACROSS SAID CATHODE AND ANODE AND THEREBY MAINTAINING A SUBSTANTIALLY CONSTANT AVERAGE VOLTAGE BETWEEN SAID CATHODE AND SAID ANODE. 